The invention relates to electronic stethoscopes and, in particular, to a hand-held electronic stethoscope having an active filter in which the center frequency and bandwidth are continuously adjustable over the entire range of somatic sounds.
Originally developed in 1816 by the French physician Rene Laennec to permit auscultation without actually touching an ear to the patient, a traditional or acoustic stethoscope gathers sound in a chest piece and conveys the sound to a physician's ears through a tube attached to the chest piece. On one side of the chest piece is a bell-shaped metal cup rimmed with rubber for transmitting sounds of lower pitch and on the other side of the chest piece is a wider, flatter cup covered with a thin, taut, plastic diaphragm for transmitting sounds of higher pitch. A tube connects the chest piece to a Y fitting for splitting the sound between two tubes having earpieces at the free ends thereof.
An acoustic stethoscope limits the mobility of a physician. The tube from the chest piece to the ear piece must be relatively short to minimize attenuation of the sound. Lack of mobility can be a critical problem, e.g. for an anesthesiologist who must either be tethered to a patient or leave the patient unmonitored while attending to other duties within an operating room. Clinical physicians often find it necessary to be away from a patient and yet still wish to monitor the patient.
An acoustic stethoscope can not adjust the loudness of the sounds and can not separate sounds of interest from other sounds, although some manufacturers adjust the thickness of the diaphragm and the shape of the bell to emphasize sounds in a particular frequency range. It was not realized at the time that Dr. Laennec had an acute sense of hearing and was able to distinguish sounds readily. Other physicians, without his aural acuity, have a more difficult time using a stethoscope to its full potential.
Electronic stethoscopes can overcome these difficulties and provide other advantages, such as powering a loudspeaker for several people to hear the sound simultaneously. An electronic stethoscope includes a microphone for converting sound waves into an electrical signal, an amplifier, and a filter to remove extraneous noises, particularly higher frequencies, from the electrical signal. The amplified, filtered signal is then coupled to small speakers in a headphone worn by the physician or is amplified further and coupled to a loudspeaker.
The filter in an electronic stethoscope has been the subject of many and varied proposals in the prior art. Some patents disclose modifying the frequency response of the filter by switching capacitors into or out of the filter, e.g. U.S. Pat. Nos. 4,534,058 (Hower); 4,528,690 (Sedgwick); and 4,254,302 (Walsh). Other patents, such as U.S. Pat. No. 4,226,248 (Manoli), disclose selecting one or more bandpass filters by means of a switch.
In all such filters, the frequency response of the filter is changed in discrete steps rather than continuously. Electronic stethoscopes tend to be application specific because the filters can not be adjusted continuously over the entire band of somatic sounds, approximately 1-3,000 hz. This range, more than ten octaves, is difficult to cover in a continuously adjustable active filter without instability, i.e. without oscillation.
While providing some enhancement of a sound of interest, the patented systems are not as flexible or as easy to use as many physicians desire. The patents listed above describe apparatus which cannot be held in one hand or fit into a pocket and the controls for which are not easily manipulated by one hand. In addition, simple filtration is often unsuited to selecting the sound of interest to a physician. For example, tailoring the frequency response of a stethoscope to heart sounds, or a particular heart sound, often means that the stethoscope is unsuited to monitoring other sounds, e.g. respiration.
Many sounds of interest are near the threshold of human hearing, i.e. below thirty hz. Bandpass filters or high pass filters may attenuate low frequency sounds below the threshold of hearing. Often, a physician wants a sound emphasized without eliminating all other sounds since the other sounds provide a frame of reference. Occasionally, a physician wants to hear sounds unfiltered, i.e. a faithful reproduction of all sounds detected by the microphone. The prior art does not address these considerations.
In view of the foregoing, it is therefore an object of the invention to provide an electronic stethoscope which improves a physician's ability to detect, isolate, amplify, and listen to the sounds produced within a patient.
Another object of the invention is to provide a portable, hand-held electronic stethoscope that can be operated by the hand supporting it.
A further object of the invention is to provide an electronic stethoscope which can transmit somatic sounds to a physician's headset or to a remote speaker.
Another object of the invention is to provide an electronic stethoscope having an improved filter in which the bandwidth and center frequency of the filter can be adjusted independently throughout the entire range of somatic sounds.
A further object of the invention is to provide an adjustable, stable, wide band, state variable filter.
Another object of the invention is to provide an adjustable, stable, wide band, Butterworth filter.
A further object of the invention is to provide an electronic stethoscope which can reproduce somatic sounds with high fidelity, by-passing the active filter.
Another object of the invention is to provide an electronic stethoscope in which the output signal is corrected for the frequency response of the human ear.